According to LTE Release 8, downlink communications from a base unit (eNB) to a wireless communication device (user equipment or “UE”) utilize orthogonal frequency division multiplexing (OFDM). In using OFDM, orthogonal subcarriers are modulated with a digital stream, which may include data, control information, or other information, to form a set of OFDM symbols. The orthogonal subcarriers may be contiguous or non-contiguous frequency bands, and the downlink data modulation may be performed using quadrature phase shift-keying (QPSK), 16-ary quadrature amplitude modulation (16QAM), or 64QAM.
Fourteen OFDM symbols are configured into a one millisecond (1 ms) downlink subframe for transmission from the base unit in the normal Cyclic Prefix (CP) case (and twelve OFDM symbols for the extended CP case). Within a subframe, data from a serving base unit is transmitted to its UEs on a Physical Downlink Shared CHannel (PDSCH) and control information is signaled on a Physical Downlink Control CHannel (PDCCH).
Control information in the PDCCH is transmitted using scheduling messages of different predefined downlink control information (DCI) Formats. These scheduling messages inform a UE of the downlink control information (e.g., modulation and coding scheme, transport block size and location, pre-coding information, hybrid-ARQ (HARQ) information, UE identifier, etc.) that is required to decode the downlink data transmissions in the PDSCH or to transmit the uplink data on the Physical Uplink Shared CHannel (PUSCH). This control information is protected by channel coding (typically, a cyclic-redundancy check (CRC) code for error detection and convolutional encoding for error correction), and the resulting encoded bits are mapped on the time-frequency resources of the downlink subframe.
The smallest time-frequency resource unit for transmissions is denoted a resource element (RE), which is one OFDM symbol (smallest time unit) by one subcarrier (smallest frequency unit). A group of four REs (or four REs plus two reference signal REs) is called a resource element group (REG). Nine REGs can make a Control Channel Element (CCE). The encoded PDCCH bits are typically mapped onto 1, 2, 4, or 8 CCEs, which are referred to as aggregation levels 1, 2, 4, and 8.
The UE searches different hypotheses (i.e., hypotheses on the aggregation level, DCI Format size, etc.) by attempting to decode downlink transmissions using a finite number of allowable configurations. This process is referred to as “blind decoding”. For example, a UE performs blind decoding using the starting CCE locations allowed for that particular UE. This UE-specific search space is typically configured during initial set-up of a radio link and can be modified using a Radio Resource Control (RRC) message. Similarly, a common search space is also defined that is valid for all UEs being served by the same eNB and might be used to schedule broadcast downlink information like Paging, Random Access Response, or others.
A particular UE must locate the CCEs corresponding to each PDCCH candidate it is to monitor (i.e., blindly decode for each subframe control region). The CRC of each PDCCH is typically masked (e.g., using an exclusive-OR operation) by an identifier corresponding to the user equipment that the base unit is trying to schedule. The identifier is assigned to the UE by its serving base unit. This identifier is known as a radio network temporary identifier (RNTI). There are several types of RNTIs, such as cell RNTIs (C-RNTIs), semi-persistent scheduling RNTIs (SPS-RNTIs), and temporary cell RNTIs (TC-RNTIs). When a UE decodes a PDCCH, it must apply the appropriate RNTI in the form of a mask to the PDCCH CRC for successful PDCCH decoding to occur. When a UE successfully decodes a PDCCH of a particular DCI Format type, it uses the control information from the decoded PDCCH to determine, for example, the resource allocation, hybrid-ARQ information, and power control information for the corresponding scheduled downlink data transmission.
In addition to PDCCH signaling, a control region of a downlink subframe also includes a Physical Hybrid-ARQ Indicator CHannel (PHICH) that is used to transmit hybrid-ARQ acknowledgements, reference signals, and a Physical Control Format Indicator CHannel (PCFICH). In the context of LTE Release 8, each eNB-to-UE downlink has 1, 2, or 3 OFDM symbols at the beginning of each subframe for control signals. The number of OFDM symbols in this control region may vary each subframe and is signaled via the PCFICH in that same subframe. In some cases, the value of PCFICH may be signaled via higher layer signaling or may be fixed.
All the remaining OFDM symbols in the subframe are typically considered the data region of the subframe, and these symbols create the PDSCH. PDSCH transmissions can be mapped into one or more resource blocks (RBs). Typically, an RB is a set of subcarriers and a set of OFDM symbols. For example, an RB may contain 12 subcarriers (with a subcarrier separation of 15 kHz) and 7 OFDM symbols, with some resource elements being assigned to carry pilot signals, etc. PDSCH allocations for a UE are typically scheduled in pairs of RBs, with each RB pair spanning a single subframe and indexed using a single RB identifier.
In a heterogeneous wireless network, HeNBs (also called femto-cells, pico-cells, or closed subscriber group (CSG) cells) can be deployed to operate in regions where at least part of the femto-cell's bandwidth is shared with eNB macro-cells. Due to the shared bandwidth, this type of deployment is considered to be high risk from an interference point-of-view. For example, a downlink of a macro-cell eNB can severely interfere with a downlink of a femto-cell HeNB thereby degrading the quality of service for UEs served by either the eNB or the HeNB.
LTE Release 8 is expected to evolve into LTE-A to support spectrum aggregation wherein a base unit can transmit data on multiple component carriers in a single subframe and a user terminal can receive multiple component carriers in a single subframe. Thus, if additional component carriers are available to macro-cells, there comes an increased risk of interference due to overlapping macro-cell and femto-cell component carrier frequencies.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Drawings and accompanying Detailed Description.